During geological time periods, large masses of land are raised above sea level. Erosional forces wear these down depositing the material in low places usually filled with water. Such regions may then themselves be raised forming mountains and the erosional process is repeated.

Three distinct layers of sedimentary material carried from high regions to the west were deposited over southern Saskatchewan during the cretaceous period (60-130 million years ago).

The first of these (the Bearpaw) often exceeds 1100 feet in thickness. It forms the subtract material to the north of the Missouri Coteau and is charaterized by dark marine shale, bentonite layers, and sandstone tongues.

The second layer (the Eastend) shows a transition from marine to fresh water. It is characterized by rusty greenish to yellowish siltstones, sandstones, and mudstones. The upper layer is kaolinized.

The third layer (the white mud) is non marine and varies from 0-45 feet in thickness. The white kaolinitic material with grey to black carbonaceous layers make it easy to recognize. It is exposed in the valley near Castle Butte, Highway 34 and in the Cathedral area.

Two other layers (the Battle and the Frenchman) were deposited near the end of the cretaceous and while they were exposed in Kildeer, Twelve Mile Lake area appear to be absent in the Big Muddy. During the Paleocene, a layer (the Ravenscrag) up to 500 feet thick was deposited. It is characterized by coal beds, non marine sands, shales, silts, and clays. Where exposed on a cliff face the lower edge is grey in color and is called the "grey facies".

The upper layer is brownish often containing white kaolinitic Willow Bunch member material. This layer is called the "buff facies". In the Miocene a cabby gravel and crossbedded sand layer (the Wood Mountain) was deposited over the Ravenscrag. The ice age (Pleistocene) changed the face of this area. To the north the ice sheets were much thicker but to the south they were non existent.